Solar thermal energy collection is used commercially for hot water heating and space heating. Typically a flat plate solar collector is utilized to absorb the solar energy onto a highly conductive flat sheet-metal plate, and the solar energy is transferred from the flat plate to tubes (typically copper) containing water or other heat transfer fluid circulating through the tubes, and the heat is exchanged for heating water. Other type of solar collectors for thermal energy systems include evacuated glass tubes, wherein the heat transfer fluid-containing tubes are integrated into the evacuated glass tubes to improve radiant heat absorption efficiency by eliminating convective heat losses within the evacuated glass tubes. Evacuated tube collectors enable a higher temperature to be achieved compared to conventional flat plate collectors, but at some loss in heat transfer area relative to a flat plate collector of the same overall system physical dimensions.
Heat pumps are a common source of residential and industrial building heating. Typically a refrigerant circulating within the heat pump system is used to extract heat from ambient air, and the refrigerant undergoes evaporation, compression, and condensation to provide heat to the interior of a home or building. The heat pump is able to raise the air temperature within the home or building based on the thermodynamics of the expansion/compression/condensation process and the source air temperature to the heat pump. However it is well known that below certain ambient temperatures, a heat pump can no longer heat the ambient air to a level sufficient for a comfortable interior temperature; typically in these cases an electrical resistance heating unit must provide supplemental heating. The electrical resistance heating is very energy intensive and therefore expensive to operate as well as a major consumer of energy. At some other ranges of ambient temperatures, a heat pump can function without the electrical resistance heating but only at interior thermostat temperature settings that some people would find lower than desirable. Due to the thermodynamic laws, the heat pump is limited to a certain ability to raise the interior air temperature in the home or building compared to the outside ambient air temperature. The difference in outside ambient air temperature compared to the interior air temperature is the “delta Temperature”; the lower the ambient temperature, the lower the temperature that the air-source heat pump is capable of providing to the interior home or building. The heat pump energy efficiency also suffers when the outside air temperature is substantially lower than the desired interior air temperature; this is best expressed as the “Coefficient of Performance” (COP) which measures the amount of heat output per unit of input electrical energy to the heat pump compressor. The Coefficient of Performance of the heat pump can vary from as much as 3 to 4 units of energy output per unit of electrical input to the heat pump compressor on mild days around 10° C., to as low as 1 unit of energy output per unit of electrical input to the heat pump compressor on a cold day around −18° C.
Recent developments in improving heat pumps relate to geothermal heat pumps. These systems use the ground as a heat source, taking advantage of warmer temperatures below the ground surface to boost efficiency (improve the COP) of the heat pump by providing a source temperature to the heat pump that is higher than the ambient air temperature. However, the geothermal heat pumps require significant initial investment and usually involve substantial and expensive drilling into the ground. Further limitations of geothermal heat pumps include soil-specific performance/problems with certain soil conditions, corrosion of pipes in the ground, and possibilities of ground-water contamination from the heat-transfer fluid circulating in the underground pipes.